The volume pedal can be placed before the amp (pre) or after (post). With the volume in the Post position then both setups would behave identically: it will affect the loudness of the preset but should not change the basic tone.

With the volume pedal in Pre, the game changes ...

In the first scenario, your OD would always be before the volume pedal. Therefore the effect would be similar to having the pedal in the post position.

In the second scenario ...... well to be honest I am not 100% sure. I cannot find any reference that says the exact effects order for the HD400. I would expect that the volume control in the 'Pre' position would be the very first effect. That being so: the FX Loop is between the FX1 and FX2 blocks so therefore the level of signal being fed to the overdrive pedal would be controlled by the volume pedal. This time, the amount of distortion would be reduced along with the volume level and the tone would change. Iniitialy reducing the pedal would reduce distortion but have little effect on the volume. After a certain point, the volume would reduce also.

The information above may not be current, and you should direct questions to the current forum or review the manual.